Still, the fact that NASA and the JMA used different calculation methods to arrive at similar results is telling. Another agency, NOAA, will release its estimate of May global temperatures next Monday using a third method.

The exceptionally warm month was egged on by a building El Niño in the tropical Pacific. Europe, Central Asia, Russia, Japan, Australia, Southern Africa, Brazil, Eastern Canada, and Western North America (so, basically everywhere) were warmer than normal last month. Besides shifting weather patterns worldwide, that extra El Niño-fueled ocean warmth is supercharging the Earth’s temperature, which could help make 2014 the warmest year since human records have been kept, and probably for much longer. El Niño’s effects should peak between October and December and will likely continue into early 2015.

NASA uses a baseline set from 1951-1980 to determine how much a month’s temperature deviates from normal. The agency’s preliminary numbers show the combined global land and ocean temperature during May deviated from that baseline by about three-quarters of a degree Celsius, the most of any May since accurate records began in 1880. The 0.76 degree Celsius anomaly is tied for the sixth-largest anomaly for any month since 1880. All 10 of the biggest monthly temperature anomalies on record have occurred since 1995, according to NASA:

January 2007: 0.93
March 2002: 0.89
March 2010: 0.87
February 1998: 0.86
April 2010: 0.82
October 2005: 0.76
May 2014: 0.76*
February 1995: 0.75
June 1998: 0.75
November 2013: 0.75
*Preliminary

Update, June 23, 2014: On Monday, new NOAA data joined NASA and JMA in declaring that May 2014 was the warmest May on record. The global record was driven primarily by excessively warm ocean temperatures that NOAA ranked as some of the warmest that had ever been measured, "tying with June 1998, October 2003, and July 2009 as the highest departure from average for any month on record."

Four of the five warmest Mays on record have occurred in the past five years: 2010 (second warmest), 2012 (third warmest), 2013 (fifth warmest), and 2014 (warmest); currently, 1998 has the fourth warmest May on record.

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